In recent years, as the regulation for exhaust gas has become stronger, honeycomb structures having a higher purification ability for exhaust gas have come to be required, and it is being actively tried to improve the warm-up property connected with the purification ability, of a honeycomb structure having a catalyst loaded on the partition walls, by making smaller the thicknesses of the catalyst-loaded partition walls and consequently reducing the heat capacity of the catalyst. The requirement for improvement of purification ability is increasing year by year and honeycomb structures having wall thicknesses as small as about 0.1 to 0.2 mm are currently in use as a mainstream while honeycomb structures having wall thicknesses of even 0.1 mm or less have come to be used partially. Such requirement for thin partition wall is considered to become even severer in future.
It is also being actively tried to mount a converter system comprising a honeycomb structure having a catalyst loaded on the partition walls, right below an exhaust manifold to quickly activate the catalyst loaded on the partition walls of the honeycomb structure, by a high-temperature exhaust gas and reduce the amount of a harmful substance emitted right after engine start.
When a converter system comprising a honeycomb structure employing thin partition walls is mounted right below an exhaust manifold in response to such a requirement, however, there arises an erosion phenomenon that the end face of honeycomb structure is scooped out by foreign matter in exhaust gas (which is mostly particulate substances of various particle diameters, derived from, for example, a material composing the exhaust manifold and a welding material used in production of the exhaust manifold). This erosion phenomenon has become an important problem.
This erosion phenomenon is important for the following reasons. The erosion phenomenon invites the detaching of catalyst at the exhaust gas-injected end face of honeycomb structure and subsequent reduction in catalytic activity; in some cases, the erosion proceeds rapidly, does not stay at the vicinity of honeycomb structure end face alone, and develops deep into the honeycomb inside, leading to the fracture of honeycomb structure. Further, when the outer peripheral portion of honeycomb structure and its vicinity are damaged severely by the erosion phenomenon, the mat material used for holding the honeycomb structure inside a converter case is exposed; as a result, the mat material is scattered by injection of exhaust gas, and the scattered mat material may become foreign matter and further damage the honeycomb structure.
Such an erosion phenomenon is attributed to the above-mentioned recent years' requirements, specifically the matters described in the following (1) and (2). A means is strongly needed which satisfies requirements of improvement in warm-up property connected with purification ability and reduction in the amount of harmful substance emitted right after engine start and yet can give a higher erosion resistance.
(1) Ordinarily, in a honeycomb structure having thin partition walls, the fracture strength to external force is low and, in particular, the fracture strength of each partition wall is influenced directly by the wall thickness. Therefore, a smaller wall thickness results in a lower fracture strength of each partition wall.
(2) When a converter system is mounted right below an exhaust manifold, as compared to conventional below-floor mounting, the honeycomb structure thereof is exposed to an exhaust gas of higher temperature and higher pressure. Therefore, the honeycomb structure undergoes a lager thermal load (e.g. a larger thermal shock) and moreover tends to be influenced by the drifting and pulsation of exhaust gas.
In order to increase the strength of honeycomb structure, there was proposed a honeycomb structure wherein relatively thick partition walls and relatively thin partition walls are provided uniformly [JP(U)-A-58-19743]. There was also proposed a honeycomb structure wherein the thicknesses of partition walls are made smaller regularly towards the center of honeycomb structure to ensure the fracture strength of whole honeycomb structure to external force and further there are employed a larger contact area and a shorter warm-up time to improve the purification ability (JP-A-54-110189).
These honeycomb structures, however, were produced for below-floor mounting, and no consideration was made to the above-mentioned recent years' requirements of thinner partition walls of honeycomb structure and mounting of converter system right below exhaust manifold, or to the erosion phenomenon arising in connection with these requirements.
Therefore, these honeycomb structures were unable to actually improve the warm-up property connected with purification ability while exhibiting a high erosion resistance.
Further, in the honeycomb structure of JP(U)-A-58-19743, since relatively thick partition walls and relatively thin partition walls are provided uniformly, the strength of whole structure is made larger but there was also a problem of reduced thermal shock resistance.
Meanwhile, there was proposed a honeycomb structure wherein the thickness of partition walls is made larger only at the structure end at which an exhaust gas is introduced, for improvement in erosion resistance (JP-A-2000-51710).
In this honeycomb structure as well, no consideration was made to that the erosion phenomenon arising in connection with the above-mentioned recent years' requirements of thinner partition walls of honeycomb structure and mounting of converter system right below exhaust manifold is seen mostly only at the particular area of honeycomb structure end.
Further, since each partition wall has a thick portion and a thin portion pr se, a thermal stress tends to concentrate at the boundary of the thick portion and the thin portion and therefore no sufficient thermal shock resistance could be obtained.
The present invention has been made in view of the above problems. The present invention aims at providing a honeycomb structure, etc. all of which satisfy recent years' requirements of improvement in warm-up property connected with purification ability and reduction in the amount of harmful substance emitted right after engine start and yet have a sufficient mechanical strength to external pressure, a high erosion resistance and a high thermal shock resistance.